Wave transmission with narrowed band



Feb. 27, 1934. J, c. STEINBERG WAVE TRANSMISSION WITH NAHROWED BANDFiled '.Jne 10 1932 from instant to instant.

Patented Feb. 27, 1934 PATENT @Fries WAVE TRANSMISSION WITH NARROWEDBAND John C. Steinberg, Sparta, N. J., assigner to Bell TelephoneLaboratories, Incorporated, New York, N. Y., a corporation of New YorkApplication June 10, 1932. Serial No. 616,435

s claims. (ci. 17a-44) The present invention relates to a method andsystem for transmitting intelligence in" which the total frequency rangerequired is less than that embraced in the intelligence-conveying wavesas produced.

The invention will be described with particular reference to speechwaves, although the invention is equally applicable to other kinds ofwaves such as music as well as waves used to produce visual elects orthe like.

The invention depends upon the existence of` certain statisticalcharacteristics in the waves, which enable their subdivision intocomponents of diierent amplitude and frequency level, varying Thecomponents occurring at any one small' instant of that time do notfill-the entire frequency range occupied by the waves occurring over aconsiderable time. Advantage of this fact is taken by shifting thefrequency level of certain components where they exist substantiallyalone at any instant so that they occupy the same frequency level as (orat least overlap in frequency) the components of other frequenciesexisting substantially alone at some other instant of time.

In the case of speech, it has been determined that the lower portion ofthe total frequency range is predominantly occupied by the vowel andsemi-vowel sounds. The upper portion is occupied principally by the stopand fricative consonant sounds. Moreover, both classes of sounds do notexist in speech in the same instant of time but they occur in rapidsuccession. This. fact makes it possible to divide the one class ofsounds from the other on a time basis. Also the amplitudes of the voweland semi-vowel sounds are larger than those of the stop and fricativeconsonants. The two classes can, therefore, be distinguished from eachother in time, in frequency range and in amplitude or energy.

In practicing the invention, lters may be used to separate the lowfrequency-large energy components from the high frequency-low energycomponents. A device responding differently to these two classes ofcomponents, switches one or other of the lters into circuit. One filterpasses the components directly to the line. The other leads to afrequency-shifting circuit `such as a modulator which changes thefrequency level of these components so that they occupy the same rangeas those passed by the other lter, or at least overlap that range. Thusboth classes of sounds are transmitted in a reduced band width.

At the receiver a second device differently responsive to the twoclasses of wave components controls connection of the line to a pair ofcircuits one of which leads directly to the receiver and the other to afrequency shifting circuit which eifects a change opposite to thatproduced in 60 these components at the transmitter.

A system of this general character is disclosed in my Patent 1,836,824,granted December 15, 1931. The present invention comprises analternative method and means of distinguishing vowel o5 and semi-vowelsounds from consonant sounds. The invention will be illustrated anddescribed as embodied in a speech transmission system which is identicalwith that disclosed in my patent supra except for the means employed todistinguish between the two classes of speech components. The inventionis, of course, capable of other embodiments and may be used, for example, merely to effect separation of the vowel and semi-vowel soundsfrom the consonant sounds without regard to the use for which suchseparation is made.

In accordance with the present-invention. that characteristic of speechis made use of which is exhibited in the fact that vowel and semi-vowelsounds contain large components which are harmonies of a fundamentalfrequency whereas stop and fricative consonant sounds either are lackingin these harmonics or contain them only in a much smaller degree.

The wave forms of the vowels and semi-vowels show that they arecharacterized by certain transient and resonant variations that areimpressed on a direct stream of air that has been modulated with thefundamental or vocal cord tone. This no fundamental in the case of a.mans voice is of the order of 125 cycles a second while in a woman'svoice it is around 250 cycles. The prominent harmonies lie in thefrequency range betweenv 300 and 2000 cycles.

The wave forms of the stop and fricative consonants show that they alsoare characterized by transient and resonant variations which, however,are impressed on a direct stream of alr which appears to be modulatedwith variations 100 that arise from the frictional effects of thepassage of air through the vocal cavities. In the case of the voicedstop and fricative consonants, the fundamental or vocal cord tone isalso present to a small extent, but there is a noticeable ab- 105 senceof overtones of the fundamental in the frequency range from 300 to 2000cycles.

The semi-vowel sounds which contain prominent harmonics in this rangeand thus resemble the vowels are l. 1n, n. no. r, w. and y. vThe unnoland will be of insufiicient voiced stop and fricative consonants are p,k, t, ch, (chin) and f, s, sh, th. (thin) respectively. The`corresponding voiced consonants which, as stated, are weak in harmonicsof the fundamental in the region 300 to 2000 cycles, are b, g, d, gi,and v, z, zh (azure), th (then). These latter are thus classed with the.corresponding unvoiced consonants as regards lack of prominent harmonicsin the range mentioned.

In accordance with the present invention the two classes of sounds(vowels and semi-vowels as against stop and fricative consonants) areseparated by detecting or rectifying speech waves in the region fromabout =300 cycles up to the order of 1700 to 2000 cycles and passing theresult- `ant detectedcomponents through a filter with a pass range fromabout 100 to 300 cycles. The sounds that have prominent over-tones of afundamental of either 125 cycles or .250 cycles will upon vdetectionyield a considerable amount of such fundamental, which will pass throughthe lter and may be used to operate suitable switching or selectingapparatus. Sounds that are weak or lacking in these harmonics willproduce little or no component in the 100l to 300 cycle range strengthto operate the switching device.

The invention will now he described as embodied in a two-way telephonesystem shown schematically in the single iigure of the accompanyingdrawing, which as stated is identical with 3 of my patent supra exceptfor the switch control circuits.

The drawing shows a two-way terminal circuit for interconnecting thesubscribers line L1 at the left with the main telephone line L2 shown atthe right.

It is assumed in this illustrative example that the maximum frequencywhich it is practicable to transmit-over the line La is about 2250cycles. Of course, speech could be transmitted over this line employingthe range from about 250 cycles to 2250 cycles and commercial qualityfor many purposes would be obtained. However, in case it is desired toobtain a higher quality of transmission than would be possible with therange 250 to 2250 cycles by direct transmission, the

method of the present invention permitting utilization of the range 250to 3250 cycles per second will be described showing how an increase inquality i'or this available total line range can be obtained.

The subscribers line L1 and the telephone line La are each shown asprovided with the usual hybrid coils H1 and H2 respectively, and theusual line balancing net works L1N and LzN respectively.

For transmitting from the line L1 to the line La, there is provided apath generally indicated by T and for transmitting in the oppositedirection between Ia and L1 there is provided a second path generallyindicated by R.

In the transmitting path T beginning at the hybrid coil H1 there is rsta volume control circuit for reducing to the same level speech wavesreceived over different length subscribers lines or from individualshaving different voice strength. This volume control may be of anysuitable type, either automatic or manual, but preferably automatic,examples of this type being given in patents of R. C. Mathes No.1,810,025 granted June 16, 1931, and D. Mitchell No. 1,853,- 070 grantedApril 12, 1932. 'Ihis volume control when used in the present systemwill have va relatively large time lag so that its adjustment will notbe changed for instantaneous variations in speech level but so that itwill regulate only the average speech level over a considerable periodof time.

On the output side of the volume control 10 the circuit branches intothe two portions 11 and 12.

Branch 11 leads through a lter 35, detector 36, lter 37 and rectifier 38to the switching relay 14. Filter 35 has a pass range from about 300 to1700 cycles, suilicient to embrace the principal harmonics of thefundamental or vocal cord frequency. Strong harmonics in this range areacted upon in detector 36 to yield an output component of fundamentalfrequency which is selectively passed through filter 37 and upon beingrectified at 38, causes operation of relay 14. Suitable amplification(not shown) may be used in branch 1l. Thus vowel and-semi-vowel soundswill cause actuation of relay 14 while consonant sounds will fail tooperate the relay.

Circuit 12 leads directly to filter 15, through the normally closed backcontact of relay 14. This filter 15 is in the consonant branch, whichbranch is closed unless relay 14 is actuated by vowel sounds. Filter 15is designed to pass currents of frequencies between 1250 cycles persecond and 3250 cycles per second but to suppress currents of both lowerand higher frequencies. This and the other filters throughout the systemmay be designed inaccordance with the principles laid down in the U. S.patent to G. A. Campbell 1,227,113, patented May 22, 1917.

The output side of lter 15 is connected to modulator 16 which ispreferably of the type disclosed in U. S. patent to Carson 1,343,306,dated June 15, 1920. This modulator is supplied with carrier waves fromsource 17, these waves having a frequency of 20,000 cycles per second.This source is preferably a vacuum tube oscillator such as is commonlyused in carrier wave systems. As explained in the Carson patent, thistype of'modulator prevents the unmodulated carrier components from thesource I7'from passing into the outgoing circuit so that only the twoside bands resulting from the modulating action of the circuit appear inthe final output.

'I'he output of modulator 16 lis connected through a second illter 18which is designed to pas only the range from 21,250 to 23,250 cycles persecond, this being the upper side band resulting from the modulation incircuit 16. This side band is then demodulated in circuit 19 by means of a sustained wave of 21,000 cycle frequency generated at source 20.'Ihis demodulator 19 may be identical with the modulator circuit 16 andis of the type disclosed in the Carson patent referred to. As the resultof the demodulation in circuit 19, a lower side band extending from 250cycles to 2,250 cycles per second is produced and this passes throughthe fllter 21, the other components of modulation being outside thetransmission range of the filter.' and therefore suppressed. y

As a result of the steps performed by the apparatus 15 to 21inclusive,it will be seen that the consonant frequencies in the range1,250 to 3,250 cycles per second have been stepped down in frequency tooccupy the range 250 to 2,250 cycles per second.

When relay\14 attracts itsarmature, in response to vowel components,circuit 12 is connected to the input of lter 22 which passes the range250 to 2,250 cycles per second directly to the outgoing circuit 23.-After suitable amplification, the currents in circuit 23 aretransmitted o income by way of vhybrid coil Hz into the outgoing lineLe.

'The apparatus in the receiving side R is closely analogous to that in'the transmitting branch 'I' and the description will follow readilyfrom that which has been given for branch T.

In the'receiving branch, the elements 45, 00, 47 and 48 are analogous toelements 35. 30, 37 and 38, already described, and function in similarmodulators 16 and 3l, and similarly, one source would be used in placeof the two sources 20 and 29 shown on `the drawing. Filter 3e may beidentical with iilter 22.

The operation of the circuit is as follows. Speech waves coming in overthe subscribers line L1 pass through the volume'control 10 and arereduced under varying conditions of service to the same volume level.These waves pass in part into the branch 12 and in part into the'branch11. 1f the waves in branch 11 contain insufdcient harmonic component toyield a requisite level of fundamental, relay 14 remains unactu= yatedand branch 12 remains connected to the lter 15. .As explained above, therelay le is adjusted so that it attracts its armature in response tovowel and semi-vowel sounds but does not attract its armature inresponse to consonant wavesin the circuit 11. Consequently the consonantcomponents in the subscribers speech occupying the range from 1,250cycles to 3,250 cycles per second pass freely through the iilter 15 andinto the modulator 16 where they modulate the sustained waves of 20,000cycles from the source 17. The upper side band resulting from thismodulation is passed through the illter 10 to .the demodulator 19 whereit is combined with the sustained wave of 21,000 cycle frequency fromthe source 20. The lower side band of this demodulating process lies inthe range from 250 to 2,500 cycles per second and therefore passesfreely through the filter 21- and through the outgoing amplifier intothe hybrid coil H2 and the outgoing line La. As a result of this processthe consonant sounds which normally occupy the range from 1,250 to 3,250cycles per second pass into the outgoing linel Lc in the reducedfrequency range extending from250 -to 2,250 cycles persecond.

Vowel sound components and semi-vowel components present in thesubscrlbers speech passing into the circuit 11v cause actuation of relay14 and shift the circuit 12 from the input of nl ter 15 to the input ofiilter 22 so that these vowel.

sounds and semi-vowel sounds which occupy the range from 250 to 2,250cycles per second pass directly into the outgoingv branch 23 and sotoune La.

Since normally e'ach word oi speech will comprise partly vowel orsemi-vowel sounds and consonant sounds, the' relay 14 will be operatingcontinually during sustained speech and for this reason its operationshould bemade as fast as practicable in order not to seriously clip anyportions of the words or syllables being spoken.

of the From the foregoing description it Will be evident that the voweland the consonant sounds transmitted to the line Lz both occupy the samefrequency range and it is necessary therefore to separate the vowelsounds from the consonant sounds in order to receive the speech inintelligible manner.

The station at the opposite end of the line L2 may be an exact duplicateof the station shown on the drawing, so that the mannerin which thespeech waves are received at the distant terminal will be evident fromconsidering that speech waves are coming in on the line L2 to thestation shownin the drawing after having been treated at the distantstation in exactly the same manner as has been described above.

As already stated, consonant over L2 from the distant station containinsuiiicient harmonics of the fundamental voice component to causeoperation-of relay 26. lThese sounds therefore pass directly into thelter 2'! and from there .into the modulator 20 where they are stepped upin frequency by combining with the wave source 20. The upper side bandresulting from this modulation is passed through the illter 30 and isdemodulated with waves from the source 32 of 20,000 cycle frequencyproducing a side band. extending from 1,250 to 3,250 cycles per second.This side band is transmitted to the n lter 33 and into the'line L1. Asa result oi the modulating actionjust described, it is the consonantsounds which were transmitted at reduced frequency level over the lineLa are elesounds' arriving oi 21,000 cycle frequency from seen that aretransmitted into the subscribers lne Li.

Vowel and semi-vowel lsound components received over the line L2 actuatethe relay 26 and connect the incoming circuit to the lter 34 instead orto the nlter 27. rli'hese vowel and semivowel soundcornponents occupyingthe range 250 to 2,250 cycles per second pass directly into the outgoingsubscriber's line L1.

It will be seen that a savingl in the total frequency range of 1,000cycles per second has been eilected since the components in the range250 to 3,250 cycles per second have been transmitted in vthe total rangeof 25,0 to 2,250 cycles per second the vowel and semi-vowels on the onehand and consonant sounds on the other, and the frequency level of theconsonant components has been stepped downward toincrease the overlapwith respect to the vowel sounds. It is within the invention, or course,to shift the frequency level vowel sound components to increase thenormal overlap with respect to the consonant components.

The invention is not to be construed as limited have been given by wayot example, nor to the specific circuits that have been illustrated. Forexample, if the line La were capable oftransmitting the range 250 to2,750 cycles per second, the division .might be as follows;

Vowel and semi-vowel sounds-250 to 2,050 cycles per second f Consonantsecond.

Various other frequency limits and frequency to particular frequencyvalues or ranges that uas-1.2m to 3,750 cycles per '145 divisions willoccur to any one desiring to practice the invention in connection with aparticular system or situation;

It will be noted that the present invention depends not merely uponamplitude of the speech waves but upon the presence or absence ofcertain frequency relations existing in some sounds and not in others.The existence or non-existence of these frequency relations isindependent of absolute volume. The method of the present inventionoffers an advantage in distinguishing from noise currents which may liein the vowel range and have large amplitude but do not exhibit thecharacteristic of having prominent components harmonically related tothe speech fundamental. The method of the invention also, and in thesame way distinguishes from strongly inflected or emphasized consonants,such as sh may have amplitudes comparable with vowel sounds but lack thecharacteristic harmonically related components of vowels. Also thevolume may vary to a greater extent than where relative amplitudes aloneare relied upon, without interfering with the operation of the methodaccording to this invention. l

Thus, in accordance with the present invention, the unvoiced consonantsounds may be amplified to any desired level before transmission so asto enable them to over-ride noise on the line. This amplification doesnot interfere with separation of ,the vowel and consonant sounds at theresonant branch at 49 for producing a controllable amount ofamplification before the line is reached. Amplification is shown also inthe receiving consonant branch at 50, in case amplifisonant soundstotheir proper amplitude level relative to the; vowels.

The element that tector (36) may be 'with a non-linear input-to-outputcharacteristic.

Instead of using an electromagnetic relay 14 to perform the switching,any other suitable means known to the artl may be used that is capableof exercising the necessary circuit control.

The means and method of the invention is not limited in its use totransmission or receiving systems but may be used to effect separationof vowel components from consonant components for any purpose.

What is claimed is:

l. The method of distinguishing vowel from consonant sound comprisingderiving fundamental wave components-from a band of speech frequenciescontaining harmonics of the fundamental but none of the fundamentalfrequency component, and utilizing the energy of said derivedfundamentalto indicate vowel sounds to the exclusion of consonantsounds.

2. The method of separating vowel from consonant sounds comprisingtransmitting speech waves over a path normally serving as the con-rsonant path, selecting from the speech waves a frequency band excludingspeech fundamental components but including prominent harmoniccomponents of the speech fundamental, deriving from the latter, waveenergy of speech fundamental frequency, and utilizing said energy whenpresent to cause the corresponding speech waves to traverse a differentpath serving as the vowel path.

3. A system for distinguishing vowel from consona'nt sounds comprisingmeans to select from speech waves a frequency band excluding componentsof fundamental speech frequency but including harmonics of thefundamental, means to derive from such -selected band a component of thespeech fundamental, and means controlled by said fundamental sounds tothe exclusion of consonants.

a. In combination, a circuit having two branches, means commutating saidtwo branches, said means normally effectively connecting said circuit toone of said branches, means to transmit speech over said circuit, filtermeans to select a frequency band from said speech excluding the speechfundamental but including harmonics thereof, means to derive a componentof speech fundamental from said selected band, and means controlled bysaid derived fundamental component for operating said commutating meansto effectively disconnect said circuit from the first it to the otherbranch.

level occupied by the other group, whereby both groups of components aretransmitted in a band of frequencies less wide than that of the speechcomponents as produced.

6. The method of separating vowel from consonant sounds comprisingexploring speech waves to determine the presence of components rich inharmonics of the fundamental speech components as distinguished fromcomponents relatively lacklng in such harmonics, and utilizing theenergy of JOHN C. STEINBERG.

so derived for indicating vowel

